Heretofore, aluminum (Al) or Al alloy has widely been used as a conductive material in silicon semiconductor integrated circuits (LSI). As manufacturing methods of LSIs make progress in fineness, copper (Cu) has increasingly been used as a conductive material to reduce the wiring resistance of interconnections and to enhance the reliability of interconnections. Cu readily diffuses into a silicon oxide film. Accordingly, a conductive barrier metal film for preventing diffusion of Cu is used on side surfaces and lower surfaces of Cu interconnections, and an insulating barrier film is used on upper surfaces of Cu interconnections. Ta or Ti, or nitrides thereof, or multilayer films thereof have been used for a barrier metal film. In order to form a Cu interconnection, there has been employed a method of pre-forming an interconnection trench (damascene trench) inside of an insulating film by dry etching and then depositing a barrier metal in the trench by a sputtering method or the like. Subsequently, a Cu layer is formed on the barrier metal by a sputtering method, and Cu is embedded into the trench by an electroplating method using the Cu layer as a seed layer (hereinafter referred to as the conventional Cu interconnection technique). Then excessive barrier metal and Cu are removed by CMP (chemical mechanical polishing) technology, thereby forming a Cu interconnection.
A Ta film or a Ta/TaN multilayer film has been put into a practical use as a barrier metal film. However, the crystal lattice unconformity between the Cu seed layer (111) and the Ta barrier metal film (110) is as high as 22%. Accordingly, a problem that the resistivity of copper is increased by electron scattering due to crystal defects at a Cu/Ta interface has become significant along with the development of the interconnection fineness.
Furthermore, the fineness in interconnection dimension has developed to dimensions of 100 nm or less in recent years. Accordingly, a method of forming a Cu seed layer by a sputtering method has difficulty in embedding Cu without generation of voids because the side coverage of a Cu film formed on sidewalls of a trench tends to be insufficient. Moreover, a Cu seed layer needs to be formed thinly in fine trenches and vias. However, there is a problem that a thinly formed Cu film is likely to cohere. Accordingly, a direct plating method, in which plating is conducted without forming a Cu seed layer on a barrier metal, has been examined for use. In order to implement such a direct plating method using no Cu seed layer, some technology is required to maintain a low resistivity of what is called a barrier metal layer and to conduct electric-field plating with high uniformity. Furthermore, in a direct plating method, a surface of a barrier metal is exposed directly to the atmosphere or a plating liquid. Thus, a barrier metal needs to have a high oxidation resistance. Accordingly, there has been examined use of a noble metal material, such as Ru, which has a lower resistivity and higher oxidation resistance as compared to conventionally-used materials such as Ta, Ti, and nitrides thereof. With regard to a direct plating method on a noble metal film, Japanese laid-open patent publication No. 2006-120870 (Patent Document 1) discloses a method of forming a conductive film that is insoluble in an electrolytic plating liquid for deposition of an interconnection material, on a surface of a substrate having recesses for interconnections formed in an insulating film, and depositing the interconnection material on a surface of the conductive film while embedding the interconnection material in the recesses for the interconnections by an electroplating method using the conductive film as a seed film. Here, the conductive film that is insoluble in an electrolytic plating liquid includes one of palladium, rhodium, and ruthenium. On the other hand, with regard to a method of employing a noble metal liner film in the conventional Cu interconnection technique, Japanese laid-open patent publication No. 2002-075994 (Patent Document 2) discloses a technique of using Ru or an oxide thereof as a barrier metal. Japanese laid-open patent publication No. 2004-31866 (Patent Document 3) discloses a technique of using a multilayer film of Ru or a nitride thereof as a barrier metal in order to prevent diffusion of Cu and improve the adhesion with Cu.